In a low pressure gradient liquid chromatography system, two or more liquids are combined in proportions according to a defined gradient composition. The contribution of the liquids to the flow is controlled using a set of valves with each liquid being controlled by a single one of the valves. Each liquid travels from the respective valve through a fluid channel to a cross connection in the system where the liquids are combined into a single flow having the specified gradient composition. The static volume in the fluid channel between a valve and the cross connection can be a source of contamination when the liquid in that fluid channel is not intended for use.
The liquid chromatography system may be operated in isocratic mode by maintaining a single valve in an open state while keeping the other valves closed. If the system remains idle for an extended period of time, liquids disposed in the unused fluid channels between each valve and the cross connection can migrate past the cross connection and into a downstream fluid channel that contains a different liquid. Subsequent operation of the system with the contaminated mobile phase can result in noise and drift in the isocratic chromatographs, and may reduce the accuracy of measurement data.